Priorities are claimed under 35 U.S.C. § 119 to Japanese Patent Applications Nos. 2002-234981 filed Aug. 12, 2002 and 2003-199269 filed Jul. 18, 2003, which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a polarization maintaining optical fiber coupler and, in particular, to a polarization maintaining optical fiber coupler that reduces loss generated in a fusion and extension process of a fused and extended portion.
2. Description of the Related Art
Various types of polarization maintaining optical fiber couplers have been proposed, and the polarization maintaining and absorption reducing (PANDA) type optical fiber is typical.
FIG. 18 is a cross-sectional view showing an example of a PANDA type optical fiber. This PANDA type optical fiber 10 is formed with a core 11 provided in the center of the optical fiber 10, cladding 12 that is provided in the shape of a concentric circle formed around the core 11 and that has a refractive index lower than that of the core 11, and two stress applying portions 13, each having a circular cross section, that are symmetrically centered around the core 11 within the cladding 12 and that have a refractive index lower than that of the cladding 12. The outer diameter of this PANDA type optical fiber 10 is approximately 125 μm.
Diameter narrowing processing is not performed on this PANDA type optical fiber 10, and the distance between adjacent outer circumferences of the two stress applying portions 13 is approximately 20 μm or less.
The stress applying portions 13 have a coefficient of thermal expansion that is greater than that of the cladding 12. Consequently, in a process to cool a PANDA type optical fiber 10 obtained by melting and drawing out an optical fiber base material, distortion is generated in the cross section of the PANDA type optical fiber 10 caused by the stress applying portions 13.
This distortion generates anisotropic distortion in the core 11. As a result, if the two orthogonal polarizations forming the light are taken as X polarization (slow polarization) and Y polarization (fast polarization), the propagation constant of the X polarization has a different value from the propagation constant of the Y polarization. Naturally, the distribution of the magnetic fields of these polarizations are also different. As a result, it is possible to obtain a characteristic in which propagation takes place in a state in which the X polarization and the Y polarization are saved.
An optical coupler manufactured using a polarization maintaining optical fiber such as the PANDA type optical fiber 10 described above is a polarization maintaining optical fiber coupler. In particular, it is known that a fused and extended type of polarization maintaining optical fiber coupler can not only be connected to external optical fibers with only a small amount of loss, but is excellent with regard to manufacturability, dependability, resistance to high optical power, and the like. This type of polarization maintaining optical fiber coupler is an effective optical component when used as an optical fiber sensor or for coherent optical communication.
FIG. 19 is a perspective view showing an example of a polarization maintaining optical fiber coupler manufactured using a normal PANDA type optical fiber.
This polarization maintaining optical fiber coupler 15 is formed by the following process. First, if necessary, a portion of a covering layer formed by plastic or the like provided on the surfaces of two PANDA type optical fibers 10 is removed therefrom. Next, the two PANDA type optical fibers 10 are adjusted and aligned such that the slow polarization axes of each are parallel. The two claddings 12 partway along the two PANDA type optical fibers 10 are then placed against each other, heated, and melted. In addition, the PANDA type optical fibers 10 are extended in the longitudinal directions to form a fused and extended portion (an optically coupled section) 14. Thereafter, if necessary, the fused and extended portion 14 is housed in a protective case (not shown) or the like that protects it from damage (see, for example, Patent Document 1 described below). Note that the slow polarization axis refers to a straight line passing through the centers of the stress applying portions 13 in each of the PANDA type optical fibers 10.
Such types of polarization maintaining optical fiber coupler include those that: (1) separate light of a specific wavelength; (2) separate and couple light of different wavelengths (for example, polarization maintaining wavelength division multiplexing (WDM) couplers and the like); and (3) separate and couple two intersecting polarization components (for example, polarization beam combiners, polarization beam splitters, and the like).
Patent Document 1—Japanese Patent Application Unexamined Publication No. 2002-323637.
Polarization maintaining optical fiber couplers manufactured by the fusion and extension of polarization maintaining optical fibers, as is described above, have a problem because excess loss tends to be generated in the fused and extended portion (i.e., loss generated in the fused and extended portion) by the manufacturing process. The problem of excess loss being easily generated is not limited to polarization maintaining optical fiber couplers that use PANDA type optical fibers, but is also generated in the same manner in polarization maintaining optical fiber couplers that use other polarization maintaining optical fibers such as Bow-Tie type optical fibers and the like.
As is described in Patent Document 1, one method of reducing this excess loss is to adjust the manufacturing conditions such that excess loss is at a minimum and then manufacture a polarization maintaining optical fiber coupler while monitoring the degree of optical coupling and the excess loss of the optical fiber couplers.
However, if excess loss increases temporarily during the fusion and extension of a polarization maintaining optical fiber, wavelength dependency of the excess loss of the polarization maintaining optical fiber coupler also increases and it becomes difficult for the polarization maintaining optical fiber coupler to be used in applications having a broad used bandwidth. Moreover, the task of adjusting manufacturing conditions such that excess loss is at a minimum in order to obtain the desired coupling characteristics is a barrier to improving productivity.
The present invention was conceived in view of the above circumstances and it is an object thereof to provide a polarization maintaining optical fiber coupler and production method for the same that allow manufacturing to be simplified and that reduce excess loss.